This invention relates to a detecting circuit such as an overcurrent detecting circuit, a temperature detecting circuit etc..
In an electric circuit, an overcurrent is often applied to the circuit on the basis of overload, short-circuit trouble, etc. In such a case, in order to protect the electric circuit and to prevent any influence upon other circuits, it is necessary to detect the overcurrent. In various controlling circuits, it is required to detect whether or not the value of a current applied to the controlling circuit is larger than a predetermined set value. Furthermore, in a temperature controlling circuit, a temperature controlled device is controlled by detecting the impedance variation of an element such as a resistor corresponding to the temperature variation of the controlled device and by comparing the detected result with a temperature set point.
In order to compare the value of a current which is applied to an electric circuit with a set point and to control the electric circuit in accordance with the comparison result, some prior art circuits are known. However, such prior art circuits have the following disadvantages. In a prior art detecting circuit for the overcurrent protection, a device such as a bimetal contact is used which utilizes heat produced by a current, an element which utilizes the electromotive force caused by the current, or a resistor in which the resistance varies in accordance with the current variation. However, since such a device has an element such as a heating member, a magnet or a resistor, power is remarkably dissipated by such an element. Furthermore, since a current to be detected is applied directly to the detecting element, it affects upon a circuit from which the current is supplied. In such case, it is impossible to isolate a control circuit from the detecting element in DC manner. Therefore, particular designs for isolation are required. Furthermore, in the prior art circuit, it is difficult to change arbitrarily a reference value, that is, a set point of a detecting current.
In a circuit such as the temperature controlling circuit which detects the impedance variation of an element corresponding to a condition of the controlled device, usually since a detected signal is small in amplitude, it is required to provide a highly sensitive amplifier and the like. In case that an analog signal detected by a circuit such as the temperature controlling circuit is processed digitally, it is required to convert the analog signal to digital form by a high accuracy analog comparator.